Method and apparatus for vapor phase soldering and cleaning products

ABSTRACT

The described cleaning of work pieces, in particular printed circuit boards or assemblies, after the soldering, is performed by washing with an inert cleansing fluid which is maintained at a temperature slightly below the melting point of the solder. As a result, solvents for dissolving the impurities and soldering fluxes which are harmful to the environment can be dispensed with. This cleaning is particularly advantageous subsequent to the vapor phase soldering since the same fluid can be used for the cleansing fluid as for the heat transfer medium in the vapor phase soldering apparatus. Therefore, the problem of an unwanted mixing of different treatment media (on the one hand heat transfer medium and on the other hand cleansing fluid) does not arise.

The invention relates to a method and an apparatus for removing solderresidues from products, in particular from printed-board assemblies orcircuit modules after the vapor phase soldering.

Although the invention is described in the following mainly asaftertreatment in connection with the vapor phase soldering ofelectronic components on printed circuit boards, the invention is notrestricted thereto. The vapor phase soldering is the preferred field ofapplication, but it is also possible to apply the cleaning measuresaccording to the invention generally to differently pretreated solderedproducts.

The U.S. Pat. No. 3,724,418 discloses a soldering apparatus in which thewater soluble solder residues can be removed from the soldered circuitboards by means of a hot water spray.

The German Offenlegungsschrift 30 06 045 discloses a method for cleaningflat products, such as printed circuit boards and flat circuit modules.According to this publication, the products are preferably upright andcontinuously transported through a chamber in horizontal direction, thecleansing fluid being simultaneously sprayed from both sides onto thesurface of the products to be cleaned by spray nozzles which aredisposed on both sides. In this connection, it is the task of the fluidjets to guide the flat products on the transport path as well as toclean them by dissolving and rinsing the impurities. The used cleansingfluid dissolves the impurities.

The German Offenlegungsschrift 30 28 325 discloses a method for cleaningprinted circuit boards wherein the substances to be removed, such assoldering fluxes, are also eliminated by a cleansing fluid spray. TheGerman Offenlegungsschrift describes various measures, such asdirection, pressure and speed of the cleansing fluid jets, in order toremove impurities also from relatively inaccessible areas.

The German Offenlegungsschrift 34 22 562 discloses a further method forcleaning printed circuit boards after the soldering. The impurities arefirst removed by washing and brushing using a liquid solvent. Thereafterthe thus cleaned printed circuit boards are dried. The used solvent iscleaned in a distilling apparatus and then used again as solvent in aclosed cycle.

The various previously known methods have in common that the cleansingfluid dissolves the impurities to be removed, such as soldering fluxes.As a result, the cleansing fluid which leaves the cleaning step ischarged with the impurities. Therefore, if the reuse of the cleansingfluid is desired, it must be cleaned before it can be employed again forthe actual cleaning of the printed circuit boards. In addition, thecleaning of circuit boards with such cleansing fluids causes substantialproblems particularly in the case of printed circuit boards with a highcomponent density. This can also be taken from the above mentionedpublications. This problem has recently been further aggravated by thefact that the increasingly used surface mounted devices (SMD) are invery close contact to the printed circuit board and the connections arein many cases soldered to the printed circuit board directly below theactual component. This narrow space makes it extremely difficult, if notimpossible, to transport the conventionally used cleansing fluid to thesoldering joints in sufficient quantity to dissolve the impurities andto carry the fluid off as fast as possible. This problem has so far beensolved by using very strong and thus aggressive cleansing fluids whichin turn attack the components and the printed circuit boards in anunacceptable manner. The fluorocarbons used in known cleaning devicesescape during operation and are harmful to the environment.

A further problem would arise if the known cleaning apparatuses wereoperated immediately after the vapor phase soldering apparatuses. Ifthese two devices are successively operated in a continuous operation, acertain amount of the heat transfer medium is carried over from thevapor phase soldering apparatus to the cleaning apparatus when theassemblies are fed into the latter and is mixed with the cleansingfluid. This additional pollution of the cleansing fluid would makeadditional cleaning measures necessary. In addition, the losses of therelatively expensive heat transfer medium emitted from the vapor phasesoldering apparatus would be substantial.

It is known from German Offenlegungsschrift 35 18 405 to remove or levelpossible excess solder still in liquid form by spraying the vapor phasemedium onto it during the vapor phase soldering after the application ofthe solder.

Due to the fact that the solder is still liquid, there is the risk ofthe components being displaced by the spraying. Said publication doesnot mention the removal of solder residues, i.e. after the solder hassolidified.

In contrast to this, the object underlying the present invention is toprovide a method and an apparatus for removing solder residues fromproducts, in particular from printed and soldered circuit boards orassemblies after the soldering process. In this connection, the use ofsolvents can be substantially dispensed with.

This object is achieved with the features of the claims.

The solution according to the invention is based on the idea to refrainas far as possible from using solvents during the cleaning. Instead, aheated cleansing fluid is used whose temperature in the cleaning step ismaintained below the melting point of the materials not to be removed,e.g. the solder. The temperature of the cleansing fluid in the cleaningstep is preferably only slightly below the melting point of the materialto be removed. Consequently, the impurities to be removed, such as thesoldering flux, are not dissolved in a solvent in the conventionalmanner but liquefied or maintained in liquid form by the heatedcleansing fluid and rinsed by the cleansing fluid. The melting point ofconventionally used solder is 183° C., hence the temperature of thecleansing fluid is maintained within a range from 150° C. to 180° C. Atemperature of approximately 175° C. has proved to be particularlyadvantageous.

It is decisive that the temperature of the cleansing fluid in thecleaning step is selected in such a way that on the one hand it is solow that the material not to be removed is not molten and that on theother hand it is high enough to liquefy or maintain the material to beremoved in liquid form.

The method according to the invention can be carried out in aparticularly advantageous manner immediately subsequent to the solderingprocess, i.e. when the soldered joints are still heated. Particularadvantages arise if the soldering is performed in a vapor phasesoldering apparatus, for instance as in the German Patent Application P38 14 870.6 of the said Applicant. In this case, the printed circuitboards which were treated in the vapor phase soldering apparatus at 215°C., for instance, can first be cooled in a controlled manner to aspecified temperature (e.g. 175° C.) below the melting point of thesolder (e.g. 183° C.) in a tempering means prior to spraying them withthe heated cleansing fluid in the actual cleaning step. In this case, itis particularly advantageous to use the same fluid for the cleansingfluid as for the heat transfer medium in the vapor phase solderingapparatus, thereby ruling out the possibility of a mutual pollution ofthe heat transfer medium and the cleansing fluid. Consequently, anadvantageous embodiment of this combination of vapor phase solderingapparatus and cleaning step is the use of a single treatment mediumwhich is vaporous in the vapor phase soldering apparatus whereas it issprayed below its boiling point and below the melting point of thesolder in the cleaning step.

Further advantageous embodiments can be taken from the subclaims and thefollowing explanation of the FIGURE.

The FIGURE shows a schematic view of a combination of a vapor phasesoldering apparatus according to the invention and a cleaning apparatusaccording to the invention.

The shown complete apparatus consisting of vapor phase solderingapparatus 2 and cleaning apparatus 1 is designed as a continuousassembly, i.e. the products to be treated are conveyed on a continuoustransport path 4 by means of work piece holders 3 from a feeding station200 of the vapor phase soldering apparatus through the latter and thenthrough the cleaning apparatus 1, to an output station 112. A possiblevapor phase soldering apparatus 2 is described in detail in the GermanPatent Application P 38 14 870.6 of the same Applicant. The products tobe treated are mounted onto work piece holders 3 in the feeding station200 and transported via lock devices 204 through the treating chamber201 of the vapor phase soldering apparatus 2 by a transport means 202.The vapor phase soldering treatment is carried out within the treatingchamber 201 in a vapor phase region 203 which is maintained for instanceat a temperature of 215° C. by means of the heat transfer medium. Thethus treated (soldered) products, such as printed circuit boards,finally exit from the vapor phase soldering apparatus 2 via a lockdevice 204' and then immediately enter the directly connected cleaningapparatus 1 (from the left in the drawing). Said lock device 204' may beformed by a liquid curtain. In this connection, the same liquid ispreferably used as the one used for the cleaning and optionally for theheat transfer. The "lock liquid" should at least be compatible with thetwo last mentioned liquids. The essential part of said cleaningapparatus 1 is the cleaning station 101 with the associated chamberhousing 102. In said cleaning station 101, the soldered assemblies aresprayed with the inert cleansing fluid whose temperature is adjusted insuch a way that it is slightly below the melting temperature of thesolder. At this temperature, for instance 175° C., the impurities on theprinted circuit board or the soldering joints, such as soldering fluxconstituents, are liquefied or maintained in liquid form and washed fromthe printed circuit board. Precision jet nozzles 106 are provided forforming the spray jets of the cleansing fluid. Said precision jetnozzles spray onto the printed circuit boards from different angles. Inorder to achieve the most optimum cleaning, several nozzles should bedirected vertically to the printed circuit boards whereas other nozzlesshould emit the cleansing fluid from different angles up to practicallyhorizontally. Due to this and the forming of very fine jets, thecleansing fluid also reaches areas of difficult access, for instancebelow SMD devices, and washes off the existing impurities at anincreased temperature.

As shown in the drawing, a tempering means 103 may be Connected prior tothe cleaning station 101 and directly behind the soldering apparatus 2,thereby cooling the assemblies which leave the soldering apparatus 2 asfast as possible and in a well defined manner to a certain temperaturebelow the melting point of the solder, for instance to 175° C. For thispurpose, a nozzle arrangement 104 may be provided in the tempering means103 which sprays a heated fluid or a heated gas onto the assemblies. Inthis connection, the temperature of the medium to be sprayed should bepreferably the same as the one of the cleansing fluid in the cleaningstep 101, i.e. for instance 175° C. The medium emitted by the nozzlearrangement 104 is sprayed at a relatively low pressure in order toavoid a displacement of the assemblies on the printed circuit boardsince the temperature of the soldering joints can still be above themelting temperature of the solder when the soldered circuit boards enterthe tempering means 103. Thus, the tempering means 103 allows thesoldered circuit boards to be carefully and rapidly cooled to atemperature below the melting point of the solder. This guarantees thatthe components remain fixed on the circuit board and cannot beinadvertently removed in the subsequent cleaning station 101 by thecleansing fluid which is emitted by the nozzles 106 at a relatively highpressure.

The fluid used for the spraying in the tempering means 103 is preferablythe same as the one used in the cleaning section 101. Consequently, noproblems can arise due to unwanted mixing of the media. Since, asalready explained above, the substance used as cleansing fluid ispreferably the same as the one used as heat transfer medium in the vaporphase soldering apparatus 2, a continuous treatment path from the vaporphase soldering apparatus 2 via the tempering means 103 and the cleaningsection 101 is obtained having a constant atmosphere so that mixingproblems are excluded from the start.

The work pieces are transported through the cleaning section 101 andoptionally through the tempering means 103 by a transport means 105,preferably on a horizontal transport path 4.

The work pieces are preferably tilted back and forth during the passagethrough the cleaning section 101 and optionally also in the region ofthe tempering means 103 and the drying section 107, thereby allowing thecleansing fluid and the soldering flux to flow off easily.Simultaneously, the direction of spray between the nozzles 106 and thework pieces varies in the cleaning section 101. This further improvesthe cleaning effect.

The exit side of the cleaning section 101 is provided with a lock gate114a in order to minimize the outward flow of cleansing fluid when theprinted circuit boards are carried out.

The cleaning section 101 should be suitably followed by a drying section107, a cooling section 110 and/or a buffer section 111 so that thepassing work pieces are cooled when they reach the output station 112.The drying process may be accelerated within the drying section 107 withthe help of a circulation blower 108. In addition, cooling devices 109or 109' may be provided in the drying section 107 and in the coolingsection 110 in order to condense the evaporating medium. The work piecesare cooled in the cooling section 110 which follows the drying section107 in the transport direction of the work pieces and are then suppliedto the output station 112 via a buffer section 111. Due to thiscontrolled cooling in the closed space of the cooling section 110 whichis substantially free of ambient air, the soldering joints are preventedfrom unwanted oxidation, on account of the reduced temperature at thesoldering joint. In addition, the lock assembly comprising the lockgates 114a to d substantially prevents cleansing fluid from flowing out.The cleansing fluid used in the various treatment sections (temperingmeans 103, cleaning section 101, drying section 107, cooling section 110and buffer section 111) is again fed to the cleaning process via arecycling system (not shown).

The output station 112 can be provided with an exhaustion device 113 inorder to exhaust and condense any Vapor remainders and feed them back tothe cleaning process via the aforementioned recycling process.

As mentioned above, the various treatment sections are separated fromeach other by locks which only open if required, optionally by anautomatic control. This substantially prevents the emission of therelatively expensive treatment media which may be harmful to theenvironment.

Since the apparatus according to the invention eliminates the need touse acids for removing solder residues, inexpensive aluminum plate canbe employed for the construction of the housing instead of high-gradesteel plate. In this connection, the aluminum plates can be screwed toeach other. This allows an inexpensive and flexible manufacture of thewhole assembly.

During the operation of a vapor phase soldering apparatus the furtherproblem arises that the work pieces are very rapidly heated when theyenter the treating chamber 201 and may be damaged since the heattransfer during the condensation of the heat transfer medium is veryhigh. It is therefore proposed according to the invention to heat thework pieces in a controlled manner prior to supplying them to thetreating chamber 201, 203, e.g. stepwise first to 180° C. and then to120° C., for instance by means of a nozzle and atomizing device similarto the tempering means 103 of the cleaning apparatus 1 (which acts,however, opposite as a cooling section). In this connection, it shouldbe noted that, according to the invention, this controlled heating priorto the vapor phase soldering can be performed with and without thedescribed cleaning process.

I claim:
 1. In a method for vapor phase soldering and removing solderresidues from products after the soldering process, in particular fromprinted-board assemblies or circuit modules, the improvement comprisingremoving solder residues from the products by washing the products withan inert cleansing fluid which is maintained as a liquid at atemperature below the melting point of the solder, and that the fluidused as cleansing fluid is the same as the one used as a heat transfermedium during the vapor phase soldering.
 2. The method according toclaim 1, characterized in that the temperature of the inert cleansingfluid during washing is maintained within a range from 150° C. to 180°C.
 3. The method according to claim 2 wherein the inert cleansing fluidduring washing is maintained at a temperature of about 175° C.
 4. Themethod according to claim 1, characterized in that the inert cleansingfluid is sprayed onto the products under pressure.
 5. The methodaccording to claim 1, characterized in that the inert cleansing fluid issprayed onto the products from different angles.
 6. The method accordingto claim 1, characterized in that the products are dried after theremoval of the solder residues.
 7. The method according to claim 6,characterized in that air is circulated in a drying station (107) forevaporating and precipitating the inert cleansing fluid.
 8. The methodaccording to claim 6, characterized in that the products are cooledafter drying.
 9. The method according to claim 1, characterized in that,prior to the cleaning, the products are treated with a spray of an inerttempering fluid at a temperature below the melting point of the solder.10. The method according to claim 9, characterized in that the fluidused as inert tempering fluid for the spray is the same as the inertcleansing fluid.
 11. The method according to claim 1, characterized inthat the solder residues are removed in a continuous operation.
 12. Themethod according to claim 1 wherein the temperature of the inertcleansing fluid during washing is slightly below the melting point ofthe solder.
 13. The method according to claim 1 wherein solder residuesare removed in a shuttle-type operation.
 14. An apparatus for vaporphase soldering and for removing solder residues from products after thesoldering process in a treatment station (201), characterized by acleaning station (101) comprising several nozzle devices (106) directedto the products to be cleaned for spraying heat transfer medium as inertcleansing fluid and by a means for adjusting the temperature of thecleansing fluid at a value below the boiling point of the inertcleansing fluid and below the melting point of the solder.
 15. Theapparatus according to claim 14, characterized by a tempering means(103) preceding the cleaning station (101).
 16. The apparatus accordingto claim 14, characterized by a drying station (107) subsequent to thecleaning station (101).
 17. The apparatus according to claim 16,characterized in that the drying station comprises a circulation blower(108).
 18. The apparatus according to claim 16, characterized in thatthe drying station (107) comprises at least one cooling device (109) forprecipitating the inert cleansing fluid.
 19. The apparatus according toclaim 16, characterized in that the cleaning station (101) or the dryingstation (107) is followed by a cooling station (110).
 20. The apparatusaccording to claim 19, characterized in that the cooling station (110)comprises a heat exchanger (109').
 21. The apparatus according to claim19, characterized in that the cooling station (110) is followed by abuffer station (111) in front of an output station (112).
 22. Theapparatus according to claim 21, characterized in that the outputstation (112) comprises an exhaustion device (113).
 23. The apparatusaccording to claim 22, characterized in that it is designed as acontinuous device.
 24. The apparatus of claim 22 characterized in thatit is designed as a shuttle-type device.
 25. The apparatus of claim 14wherein the means for adjusting the temperature of the inert cleansingfluid adjusts the temperature to a value slightly below the meltingpoint of the solder.
 26. The apparatus according to claim 14,characterized in that lock devices (114, 204) are provided between thevarious stations and at entrance and exit sides thereon.
 27. Theapparatus according to claim 26, characterized in that the lock device(204') between the treating chamber (201) and the cleaning station (101)is formed by a liquid curtain.
 28. The apparatus according to claim 27,characterized in that a fluid which is compatible with the cleansingfluid and the heat transfer medium is used for the liquid curtain. 29.The apparatus according to claim 14, characterized in that the apparatusis housed in a housing which comprises aluminum plates which are screwedto each other.